Electrical activity of the muscle layers of mammalian small intestine is being investigated. The longitudinal muscle layer gives rise to slow waves, 5-7 seconds duration and we continue to obtain evidence that these result from a rhythmic Na-K pump. We are seeking the primary oscillator in the biochemical reactions which supply ATP and are going to look for variations in ATP concentrations at peak and trough of the slow waves, also for variations in absorption at 340 nM where redox oscillations may be found. The circular muscle layer gives spikes which are based on conductance changes in calcium and we are investigating the repolarization phase. We are testing whether this is due to potassium conductance, to an active calcium extrusion and to a Ca-Na exchange. We have evidence that slow waves spread passively between the layers but are enhanced by some sort of cycling between longitudinal and circular layers. By simultaneous recording from each layer and by applying various drugs and ionic combinations we hope to discover the nature of the reinforcement. We have evidence that rhythmic spiking in the circular layer may represent a basic pattern common to all vertebrates and that this rhythmicity may be initiated by mechanical and neurohumoral stimuli. The control by intrinsic neural plexuses and transmitters will be studied in detail.